Oscillators that have a frequency determining crystal are commonly used to provide a stable output frequency. However, the crystals used in these oscillators are temperature sensitive and therefore temperature compensating means are normally required to maintain a stable oscillator output frequency.
One method of stabilizing crystal oscillator frequency, which has been used, is to enclose the oscillator within an oven and maintain the crystal at a constant temperature. This requires a large amount of space and consumes a substantial amount of power. Another method involves generating a temperature varying voltage and applying it across a voltage variable capacitor (e.g., varactor diode) to control the resonant frequency of the crystal oscillator.
In many oscillators, an AT cut crystal is commonly used and has a generally cubic frequency versus temperature characteristic with an inflection point at approximately 28 degrees C. The precise frequency versus temperature characteristics of individual AT cut crystals is variable, and depends on how the crystal was made. Thus, in order to accurately compensate an oscillator using an AT cut crystal, the voltage applied to the varactor diode should have a temperature variation which is substantially similar to that of the particular crystal being used.
Modern communication systems, including particularly cellular telephones and two-way radios, require products that are smaller, lighter and need to have extended operating times (or battery lives). A significant portion of the size and weight of a portable radio product is attributable to its power source or battery. In order to reduce the size and weight of the battery, circuitry is needed to operate at lower voltages, such as from about 2.7 to about 3.3 volts, with the total current drain being minimized.
Miniature temperature compensated crystal oscillators (TCXOs) commonly use frequency-temperature compensation schemes that generate a voltage that varies with temperature, to drive a voltage variable reactance device, in order to obtain the desired frequency-temperature characteristic. It is necessary to operate the TCXO at various voltages, while maintaining the required temperature compensation characteristics.
Traditionally, the regulated supply voltage for compensation, is obtained by utilizing a simple circuit operating directly from the supply voltage. Thus, if the supply voltage were between 4.5 and 5 volts, a regulator of approximately 4 volts would be utilized. When using reduced voltages, such as 3 volt batteries, it would be very difficult or nearly impossible to compensate the TCXO appropriately, as the circuit needs to have the capability to adjust for a range of frequency-temperature characteristics, and the adjust range is dependent on the supply voltage available to the compensation (i.e., 0-4v).
Thus, it would be considered an improvement in the art to have the capability of using a low voltage power supply, such as a 3 volt battery, which is capable of powering the temperature compensation of a TCXO. It is also considered an improvement, to minimize current drain for substantially the entire compensation circuit, by having the majority of the associated circuitry operate at a reduced supply voltage.
Additionally, not all communication systems can be converted to lower supply voltages, so TCXO designs should have the capability of operating at higher supply voltages, without requiring additional current. Economies of scale, particularly, where custom integrated circuits are developed, require that a single design can be used to operate over extended voltage ranges, or can be operated with 3v or 5v batteries.
It is also considered an improvement in the art, if certain components, such as a programmable DC to DC converter network could be utilized in a TCXO, to provide the capability of operating over an extended voltage range, by increasing the supply voltage to a level necessary to operate a variable reactance device with a minimum increase in current, by supplying that voltage (higher voltage), only to the necessary circuitry, for example, such as to a level shifting network and the variable reactance device, to control the TCXO.
It would also be considered an improvement in the art, to provide a TCXO which draws low power and current, which is adjustable to fit most crystal oscillators, and which is readily manufacturable in an integrated circuit (IC) form.